The field of adaptive structures has evolved the integration of actuation and sensing elements into structures so that the response of the system to external stimuli may be altered. Current state of the art generally involves attaching an “active material” component to a structural member composed of a linear elastic material such as aluminum or polymer matrix fiber composites. In such an arrangement the active material component can responsively and adaptively actuate the linear elastic material in response to external stimuli.
Active material components are those belonging to a category of materials that change their shape or stiffness in response to an external control field. Some examples include piezoelectric, magnetostrictive, and shape memory alloy materials. Linear elastic materials are those belonging to a category of materials where the strain and stress experienced during deformation are linearly related, and upon relief of stress, all deformation is removed.
While utility may be obtained using active materials incorporated with linear elastic materials, significant change in shape and form of the structural component is hampered by the use of linear elastic materials. These materials present a sub-optimal design choice for the design of large changes in structural shape. In one case, the structural components can be designed such that stiffness and strength are low, but the overall deformation is large. In another approach, the structure may be designed such that the system exhibits high stiffness, but as a result the reversible deformation capability of the structure is limited and large amounts of energy are required to affect a change in the geometry, along with a significant support structure to maintain the deformation. In addition, these materials cannot accomplish significant “Gaussian Curvature” or simultaneous curvature about two orthogonal axes because this requires a change in area in the plane of the deformation. Typical linear elastic materials are severely limited in their capability for changing area.
The capability for structural components to achieve large changes in shape would be greatly enabled by materials that can reversibly change their elastic stiffness. By changing elastic stiffness, the energy required for deformation can be decreased. Further advantages would result from a material, which in a lowered stiffness state could undergo large reversible deformation in at least one direction. Given these properties, it would be possible to consider making large changes in the shape of high-stiffness structural components. While this property is currently available using polymers, especially shape memory polymers, these materials possess low elastic stiffness and thus are generally not preferred for structural components.
Current structure design using static component shapes often requires compromise between various operational conditions rather than optimization over a range of conditions. If an intermittent change in structure is required, for example in the reconfiguring of an aircraft wing from a take-off or landing configuration to a cruising configuration, current solutions require extra components and added complexity is incurred. This problem is not limited to aerospace but is common to a myriad of technologies, such as for example automotive, space, telecommunications, medical, optical, or other technologies where structural or surface reconfiguration is desirable.
Other important areas where a change in stiffness is desirable include storage and deployment of expandable structures. Current methods rely heavily on complicated assemblies of rigid parts that make use of traditional mechanical components (pivots, latches, etc.). Deployable devices using variable stiffness structures would enable new designs to be considered, with fewer parts and assembles, thus reducing weight and complexity.
Other important areas where a change in stiffness is desirable includes impact/crash mitigation and protection from ballistic objects. Current approaches to each of these problems use static stiffness materials to absorb energy from impact of objects or occupants. Generally speaking there are compromises in the design of safety and armor systems due to the static materials properties. Materials which can rapidly change their stiffness and deformation properties can be used to make adaptive energy absorption components. For example, typical uses may include armor and restraint devices for vehicles.
U.S. Pat. No. 6,000,660 by Griffin, et al., herein incorporated by reference, describes a variable stiffness member which changes its stiffness by rotating an elliptical shaft, thereby changing the bending stiffness according to the change in height and width of the elliptical cross section. This concept is limited in the total change in stiffness achievable, and it is not nearly as robust as a material that exhibits intrinsic change in stiffness. In addition, this approach is not applicable to creating stiffness changes or structural shape changes in large planar surface components.
Fibrous elastic memory composite materials utilizing carbon fibers and a shape memory polymer matrix can change their elastic stiffness. Details of this approach are outlined by Campbell and Maji, in the publication “Deployment Precision and Mechanics of Elastic Memory Composites,” presented at the 44th Annual AIAA Structure, Structural Dynamics and Materials Conference, Norfolk, Va., Apr. 7-10, 2003. The limitation of these materials comes from the use of fiber as reinforcement agent, which undergo microbuckling in order to achieve high compressive strain. The mechanical properties of the composite material once microbuckling has been initiated are significantly reduced as compared to the initial aligned fiber direction. In addition, the process of microbuckling can be difficult to control in terms of the direction of the microbuckling (i.e., in-plane or out-of-plane). Another important limitation of these materials, in the case of deforming surfaces, is a limitation in the amount of area change (simultaneous strain in two orthogonal directions) that can be achieved due to the inextensibility and length of the reinforcing fibers.
Another approach intended to serve the same role as these materials, in the specific case of morphing structures, are termed compliant structures. Compliant structures employ a specific architecture that tailors deflection along a desired contour when subject to known input forces and deflections. While this approach is advantageous in some applications, it is generally limited in the number of configuration states that can be achieved and optimized. Therefore, it is not as effective when more than one pre-specified range of motion is desired.
In some applications what is needed is a structure capable of changing bending stiffness in response to a control signal such that the stiffness may be decreased, the material reshaped, and the stiffness returned after reshaping. In certain applications, what is needed is a structure that allows a change in material stiffness to permit a change in the shape and function of a structural or surface component. Further, in some applications, what is needed is a structure that exhibits increased axial stiffness. In some applications, what is needed is a structure whose resonant frequencies may be altered by changing the intrinsic structural stiffness. In still other applications, what is needed is a structure that permits single components to serve multiple roles at reduced overall complexity and weight. In yet other applications, what is needed is large reversible change in surface area of structural materials. In yet other applications what is desired is a rapidly tunable mechanical stiffness to tailor response to ballistic and crash events.